This application relates generally to correcting a static track pitch distribution and more particularly to improving a data handling device by measuring and correcting track pitch.
Disc drives are commonly used in workstations, personal computers, laptops and other data handling systems to store large amounts of data in a form that can be made selectively, readily available. In general, a disc drive comprises a magnetic disc that is rotated by a spindle motor. The surface of the disc is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. Each of the data tracks extends generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of the track on the disc surface. Typically, each data track is include a number of data sectors that store fixed sized data blocks.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to transmit an electrical signal that causes a magnetic transition on the disc surface, to write data.
As known in the art, the magnetic transducer (or head) is mounted to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disc to either read data from or write data to the preselected data track, as the disc rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disc surface due to air currents caused by rotation of the disc.
An important aspect of conventional disc drive design concerns position control of the head. A position control system is used to accurately position a head over a data track during data read and write operations. Whenever data are either written to or read from a particular data track, the transducer gap of the head should be centered over the centerline of the magnetic transitions of the data track where the data are to be written or from where the data are to be read, to assure accurate transduction of the transitions representing data. If the head is off-center, the head may transduce (i.e. either read or write, as the case may be) transitions to or from an adjacent track, and thereby corrupt the data.
A closed loop servo system is typically used to control the position of the actuator arm. In a known servo system, servo position information is recorded on the disc surface itself, and periodically read by the head for use in controlling the position of the actuator arm. Such a servo arrangement is referred to as an embedded servo system. In modern disc drive architectures utilizing an embedded servo system, each data track is divided into a number of servo sectors generally equally spaced around the circumference of the data track. Each servo sector is further divided into a servo data field which contains information for positioning the head on the user track and a user data field where user information is read or written. Typically, user information is read and written to the data track in fixed size packets called data sectors. Data sectors may be written entirely within a single user data field on a user track or they may be written to the data track in a manner such that a servo sector splits the data sector between two data fields, as is well known.
Typically each servo sector is radially aligned with corresponding servo sectors of neighboring data tracks to form a set of radially extending, spoke-like servo sections that are equally spaced from one another around the circumference of the disc surface. The equal spacing between servo sectors provides a fixed frequency of servo occurrences regardless of the radial position of the head. However, when data are recorded in a zone bit arrangement, the number of data sectors within one rotation of a disc varies from zone-to-zone, thus causing the precise locations of servo sectors of the spoke-like sections, relative to the data fields of the data sectors, to vary from zone-to-zone and within a zone.
A zone bit arrangement is a known technique to maximize the storage capacity of a disc. In accordance with the fundamental geometry of a circle, the circumferences of the data tracks increase in a direction toward the outer diameter of the disc. Thus, each succeeding data track in the radially outward direction, has more potential data storage capacity than the preceding data tracks. A zone bit recording scheme takes advantage of the increasing circumference aspect of circle geometry. In a zone bit recording, the surface of the disc is divided into a set of zones. Each zone extends for a fixed radial length, and the magnetic transition frequency is increased from zone-to-zone, in the radially outward direction. Accordingly, the number of data sectors in each track increases, from zone-to-zone, in the radially outward direction.
In an embedded servo system, each servo field contains magnetic transitions, called servo bursts, that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine bead position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo field, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disc drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
Servo bursts are typically written to the discs during the manufacture of a disc drive using a highly precise servo track writer, which utilizes the heads of the disc drive to write the servo bursts. As the servo bursts are used to define the tracks, it is important to precisely control the position of the heads as the servo patterns are written to the disc surfaces. Thus, a typical servo track writer comprises a positioning system which advances the position of the heads, a laser based position detector which detects the position of the heads and control circuitry which provides the servo information to be written to the servo fields on the discs.
Servo bursts may also be written to the disc with what is known as self-propagating servo writing. In self-propagating servo writing, the radial position signal that is used to servo-control the actuator is derived from measurements of the readback amplitude of servo bursts that were written during a previous step of the servo writing process. Thus, errors in the head position during servo writing appear as distortions away from a desired circular track shape.
As will be recognized, proper radial alignment and or spacing of the tracks on the disc is essential to facilitate reliable operation of the disc drive. While servo track writing techniques such as those described above provide a generally high degree of accuracy in radial track spacing, inaccuracies or errors in track spacing may still occur as the result of the servo writing process. In general, these track inaccuracies occur in two principle forms: dynamic or AC inaccuracies and static or DC inaccuracies. Dynamic or AC inaccuracies typically occur as a result of non-repeatable relative motion between the disc and the head during the servo writing process, which produces non-circular track shapes on the disc. In contrast, static or DC inaccuracies typically occur as the result of low frequency relative motion between the disc and the head during the track writing process and/or various inaccuracies of the track writing equipment. Whereas the effects of DC errors has in the past had minimal impact upon the operation of the servo disc drive, as higher track densities are achieved, such errors become increasingly significant.
There is a need, therefore, for an improved approach to correcting static track pitch in high performance data handling systems. It is to such a need that the present invention is directed.
The present invention comprises a method for correcting track pitch in a data handling system and an apparatus improved by that method.
In one embodiment, many parallel tracks on a storage surface of a data handling device are arranged in a circular direction. Each track has a track center defined by servo sectors written during the servo track writing process. Each successive pair of track centers has a succession of radial track spacing measured at each servo sector. The average of the succession of radial offsets between a pair of tracks is referred to as static (or DC) track spacing. Due to static track spacing errors, the static track spacing between different pairs of tracks are not equal. The static spacing of the different pairs of tracks has a statistical distribution. The smaller the variance of the distribution, the more even the track spacing. In the preferred embodiment described here a series of correction factors are used to modify the position of the head relative to the nominal track centers defined by the servo writing process. As a result, the head follows a modified (or corrected) track center. The variance of static track spacing between the corrected track centers will be significantly reduced when compared to the variance of track spacing of the original track centers.
Additional features and benefits will become apparent upon a review of the following drawings and the corresponding detailed description.